Spider Silk Spinning Silkworms
The silk used by the clothing industry is harvested from silkworms. The most famous of the silkworms being the Mulberry moth catepillar. Silk is a polymer protein composed mostly of repeating units of fibroin (2). The production of silk is not limited to silkworms. Other moth species and spiders etc. spin silk to make cocoons and webs. Silk from a variety of sources has been used for millennia. The Chinese are noted for developing the use of silk fabric. Originally, it was exclusively for royals and could only be given to others as gifts. Silk was not only used in clothing production but was used in the making of tapestries and other forms of art. Outside of China, silk was used throughout the Middle East and the rest of Asia. The advent of European exploration saw the spread of silk westward. Today, silk is still considered to be a luxury item. The Science Behind Silk Silk is synthesized by a silk gland which is a type of exocrine gland (3). The gland is a paired organ made of modified labial/salivary glands (3 ). The gland secretes silk proteins into a central duct where the proteins are 'stuck' together by the other major protein found in silk, (3). Sericin is produced by secretory cells that line the duct that the fibroin is secreted into (3). Silk has a natural shine that is attributed to the flat surfaces of the silk (2). For example, Mulberry silkworm silk has a triangular cross section and the flat surfaces reflect light creating a natural sheen (2) . Genetically Engineering Silkworms Spider silk is of great interest to researchers because of its strength and versatility. It is much stronger than silkworm silk. It has been theorised that if it became commercially available it could replace kevlar, be used to make wear-resistant clothes and could even be used in medical products like bandages and surgical thread (4). In fact, the aincient greeks used spider silk as bandages for wounds. There is no known artificial method for spinning spider silk and farming spiders for silk production poses a problem as well. Spiders will resort to cannibalism in the absence of other prey and they don't produce large amounts of silk (4). Expressing spider silk proteins in the goat is possible but the drawback there is obviously the goat's inability to make silk fibers. To circumvent these problems, Teule et al. ''(2012) transformed silkworms with chimeric silkworm/spider silk genes. They were able to show that the transformed silkworms made silk composed of a composite of the chimeric silk proteins that was stable (5) . The silk was stronger than wildtype silkworm silk and as strong as some types of spider silk (5) . To transform the silkworms, the researchers used the ''piggyBac vector system . The piggyBac vector system is a transposon system that uses a transiently expressed transposase to integrate genes of interest into TTAA sites in the host genome. Expression of the transposase is lost when that portion of the plasmid is degraded by the host cell. This allows for the stable integration of genes of interest into host organisms. The piggyBac ''vector system was used to introduce a synthetic spider silk gene in the hope that the silkworms would then produce fibers that were in part made with spider silk. Using EGFP tagged protein in some of their ''ex vivo samples allowed them to monitor expression of the protein (5) . After confirming that they did in fact have a chimeric protein, they tested the strength of the chimeric silk and compared it to wildtype silkworm silk and to spider silk (5). They found that the wildtype silk was the weakest, while the spider silk was the strongest as would be expected (5). The chimeric silk fiber samples were all stronger than the wildtype silkworm silk with one sample very closely mimicing the spider silk (5). References 1. Orth FW; A Strand of Silk. Prelinger Archives A Strand of Silk 2. Silk (2013) Wikipedia http://en.wikipedia.org/wiki/Silk 3. Mondal M, Trivedy K, & Nirmal Kumar S (2007) The silk proteins, sericin and fibroin in silkworm, Bombyx mori Linn., - a review. Caspian Journal of Environmental Science 5(2):63-76. 4. Brooks C (2013) Tangled webs: Why scientists want to recreate spider silk. in Science: Knowledge & Learning Beta (BBC Scotland, http://www.bbc.co.uk/science/0/21685308 ). 5. Teule F'', et al.'' (2012) Silkworms transformed with chimeric silkworm/spider silk genes spin composite silk fibers with improved mechanical properties. Proceedings of the National Academy of Sciences of the United States of America 109(3):923-928. 6. System Biosciences (2013) PiggyBac Transposon System Instant and Reversible Transgenesis. Vectors http://www.systembio.com/piggybac-transposon/piggybac-vectors 7. DNA2.0 (2013) piggyBac Expression Vectors. Expression Vectors. https://www.dna20.com/products/expression-vectors/piggybac